This is a phase I:II open-label study assesses the safety, efficacy, and biologic activity of BMS-188667 in patients with 10-40% body surface area of psoriasis vulgaris. BMS-188667 is an investigational drug that selectively blocks the second signal in T cell activation. Increasing evidence suggests that T-cell is involved in the pathogenesis of this disease. Patients will receive four intravenous doses of BMS-188667, and be evaluated for safety and efficacy in follow-up for a total period of 99 days.